Lithium-sulphur rechargeable batteries possess a high theoretic specific energy of 2600 Wh/kg. However, the practical specific energy of lithium-sulphur battery prototypes available today is in the range of 250-350 Wh/kg (Batteries for portable device. G. Pistoia. Elsevier 2005 P. 118; Handbook of batteries/David Linden, Thomas B. Reddy. 3d ed. P. 34.42), which is significantly lower than the theoretically anticipated value. The practical specific energy of lithium batteries is known to be 25-35% of the theoretical value. Therefore it could be expected that the practical specific energy of lithium-sulphur batteries would be about 780 Wh/kg (30% of 2600 Wh/kg). The lower value of the practically achieved specific energy of lithium-sulphur batteries as opposed to the theoretical value is determined by specific features of the electrochemical processes in lithium-sulphur batteries during their charge and discharge.
It is well known that elemental sulphur can exist in various molecular forms. Sulphur octet (S+8) is the most stable state under normal conditions. Elemental sulphur is soluble (though very poorly) in many aprotic electrolyte systems. The molecular form of sulphur in many cases is the same in both solution and in the solid state.
During discharge of lithium-sulphur batteries, the electrochemical reduction of sulphur is realised in two stages (V. S. Kolosnitsyn, E. V. Karaseva, N. V. Shakirova, D. Y. Seung, and M. D. Cho, Cycling a Sulphur Electrode in Electrolytes Based on Sulfolane and Linear Ethers (Glymes) in an LiCF3SO3 Solution//Russian Journal of Electrochemistry, Vol. 38, No. 12, 2002, pp. 1360-1363; V. S. Kolosnitsyn, E. V. Karaseva, D. Y. Seung, and M. D. Cho, Cycling a Sulphur Electrode in Mixed Electrolytes Based on Sulfolane: Effect of Ethers//Russian Journal of Electrochemistry, Vol. 38, No. 12, 2002, pp. 1314-1318; V. S. Kolosnitsyn, E. V. Karaseva, D. Y. Seung, and M. D. Cho, Cycling a Sulphur Electrode: Effect of Physicochemical Properties of Electrolyte Systems//Russian Journal of Electrochemistry, Vol. 39, No. 10, 2003, pp. 1089-1093; V. S. Kolosnitsyn, E. V. Karaseva, N. A. Amineva, and G. A. Batyrshina, Cycling Lithium-Sulphur Batteries//Russian Journal of Electrochemistry, Vol. 38, No. 3, 2002, pp. 329-331).
Reduction of elemental sulphur in octet form dissolved in the electrolyte or the reduction of sulphur-containing compounds with the production of lithium polysulphides (the compounds being well soluble in electrolytes) takes place in the first stage of the lithium-sulphur battery discharge. The first products of the sulphur octet reduction, lithium octasulphides, are not stable in many electrolyte systems. Furthermore, they undergo reactions of disproportionation with a detachment of elemental sulphur, which again undergoes electrochemical reduction. A simplified process of sulphur octet reduction can be described by the following equations:S8+2e−+2Li+→L2S8  (1)L2S8→Li2Sn+S(8−n)  (2)
However, the process of elemental sulphur reduction to lithium polysulphides is much more complicated. It is described in detail in the following papers (Margaret V. Merritt, Donald T. Sawyer. Electrochemical reduction of elemental sulphur in aprotic solvents. Formation of a stable S8− species//Inorg. Chem.-1970.-V. 9.-pp. 211-215; Robert P. Martin, William H. Doub, Jr., Julian L. Roberts, Jr., Donald T. Sawyer. Further studies of the electrochemical reduction of sulphur in aprotic solvent//Inorg. Chem.-1973.-V. 12.-pp. 1921-1925; Rauh R. D., Shuker F. S., Marston J. M., Brummer S. B. Formation if lithium polysulphides in aprotic media//J. inorg. Nucl. Chem.-1977.-V. 39.-pp. 1761-1766; Yamin H., Gorenshtein A., Penciner J., Sternberg Y., Peled E. Lithium sulphur battery. Oxidation/reduction mechanisms of polysulphides in THF solution//J. Electrochem. Soc.-1988.-V. 135.-No. 5.-pp. 1045-1048).
In the second phase of lithium-sulphur discharge, a sequential reduction of lithium polysulphides dissolved in electrolyte takes place. It takes place with a gradual shortening of the polysulphide chain, Initially to short-chain lithium-polysulphides, and further to lithium sulphide and/or disulphide as final products, these compounds being poorly soluble in electrolyte (equations 3-5):L2Sn+2e−+2Li+→Li2S↓+Li2S(n−1)  (3)Li2S(n−1)+2e−+2Li+→Li2S↓+Li2S(n−2)  (4)L2S2+2e−+2Li+→2Li2S→  (5)
In reality the electrochemical reduction mechanism of lithium polysulphides is more complicated.
The two-stage mechanism of the sulphur electrochemical reduction results in two plateaus in voltage on the charge and discharge curves for lithium-sulphur batteries. The first, higher plateau is characterized by a voltage of 2.5-2.0V relative to the lithium electrode and is explained by the reduction of elemental sulphur, while the second, lower plateau at a voltage of 2.1-1.5V is due to the reduction of lithium polysulphides.
The first voltage plateau on the charge curve (2.2-2.4V) occurs because of the oxidation of lithium sulphides and short-chain polysulphides into long-chain lithium polysulphides, while the second flat area (potentials of 2.4-2.7V) is due to the oxidation of long-chain lithium polysulphides to elemental sulphur.
At higher voltages, a higher plateau (Region A on FIG. 2, in one embodiment) is separated from a lower one (region B on FIG. 2 on FIG. 2, in another embodiment) by an inflection point
The dissolution of lithium polysulphides in electrolytes results in an increase of electrolyte conductivity when concentrations of lithium polysulphides are low, and in a significant reduction of electrolyte electroconductivity at high concentrations of lithium polysulphides (Yamin H., Peled E. Electrochemistry of a nonaqueous lithium/sulphur cell//J. of Power Sources.-1983.-V. 9.-pp. 281-287; Duck-Rye Chang, Suck-Hyun Lee, Sun-Wook Kim, Hee-Tak Kim. Binary electrolyte based on tetra(ethylene glycol) dimethyl ether and 1,3-dioxolane for lithium-sulphur battery//J. of Power Sources.-2002.-V. 112.-pp. 452-460.). Moreover, the viscosity of the electrolyte solutions increases with the increase of the lithium polysulphide concentration.
The rate and the depth of the electrochemical reduction of lithium polysulphides are significantly dependent on the physical-chemical properties of the electrolyte system. The electroconductivity and viscosity of the electrolyte has a significant influence on the reduction depth of lithium polysulphides diluted in electrolyte solutions. The reduction in electroconductivity, as well as the increase in viscosity, results in a decrease of the depth of the reduction of the lithium polysulphides. This effect is observed from the shape of the discharge curves of the lithium-sulphur cells.
The discharge curves of the lithium-sulphur cells with electrolytes of different viscosities differ significantly (Duck-Rye Chang, Suck-Hyun Lee, Sun-Wook Kim, Hee-Tak Kim Binary electrolyte based on tetra(ethylene glycol) dimethyl ether and 1,3-dioxolane for lithium-sulphur battery//J. of Power Sources.-2002.-V. 112.-pp. 452-460). The higher the viscosity of the electrolyte solution, the shorter the low-voltage plateau on the lithium-sulphur discharge curve. The low-voltage plateau may not be present at all on discharge curves of lithium-sulphur cells with very high viscosity electrolyte solutions.
Therefore, in a porous positive electrode, the increase in viscosity of electrolyte solutions in the pores (which occurs due to the dilution of lithium polysulphides following the reduction of sulphur at the first discharge stage) results in a decrease of the depth of reduction of the lithium polysulphides, and hence in a reduction of the discharge capacity of the lithium-sulphur battery.
This phenomenon is known to limit the energy density of lithium-sulphur batteries.
The degradation in electrochemical properties of lithium-sulphur cells (when electrolyte electroconductivity decreases and the viscosity increases) occurs as a result of the displacement of the electrochemical reaction from the bulk volume of the positive electrode to its surface.
The positive electrodes of lithium-sulphur batteries typically comprise microporous systems filled with electrolyte. Since sulphur and the final products of sulphur reduction (lithium sulphide and lithium disulphide) are dielectric, some electron conductive compositions are normally added to the positive “sulphur” electrodes of lithium-sulphur batteries. Most often carbon materials are used for this purpose. The electron conductive compositions or materials are in one embodiment in particulate form.
The electrochemical reduction of sulphur during discharge normally occurs on the surface of the electron conducting particles. As the process proceeds, new lithium polysulphides are diluted in electrolyte within the pores of the positive electrode. The lithium polysulphide concentration in the electrolyte increases with the discharge of the sulphur electrode. This results in a gradual increase in the viscosity and decrease in the electroconductivity of the electrolyte.
The penetration rate of an electrochemical reaction product of an electrode into the bulk, is proportional to the square root of the ratio of the electroconductivity volume to the bulk rate of the electrochemical reaction, for example as shown at (Eqn. 22.31; Electrochemical system//John Newman and Karen E. Thomas-Alyea. 3rd ed. P. 534):L/v=(RTκσ/((αa+αc)ai0F(κ+σ)))1/2 
Therefore, the decrease in electroconductivity of the electrolyte inside the pores of the positive electrode results in a displacement of the electrochemical reaction to the electrode surface and hence contributes to a decrease in sulphur utilisation (or sulphur-based compounds in embodiments of the present invention). This results in a lower specific energy of a lithium-sulphur cell.
The displacement of electrochemical reactions on to the sulphur electrode surface prevents the use of thicker porous cathodes in lithium-sulphur cells. The optimal thickness of sulphur electrodes is usually in the range of 15-30 μm, while the surface capacity is about 2-4 mAh/cm2.
Poor electrochemical performance of thick sulphur electrodes is viewed as an obstacle to gaining higher specific energies in lithium-sulphur cells due to the disproportionately high contribution of auxiliary cell components to the total weight of a cell (current collectors, electrode tabs, separator, electrolyte, etc).
Some improvement of the electrochemical properties of the sulphur electrodes (increase in the penetration rate of electrochemical reaction into the bulk of an electrode) and hence improvement in capacity can be obtained by increasing the electrolyte content of a lithium-sulphur cell. When the amount of electrolyte is increased, high viscosity concentrated solutions of polysulphides in electrolyte do not form. Moreover, sulphur is completely reduced to its polysulphide forms. However, this also represents a significant compromise as the increased amount of electrolyte contributes additionally to the weight of the battery and does not provide any significant gain in the specific energy.